Well from my knowledge The Assassin's religion isn't 'exactly' a religion based upon today's christanity or any other religion, but more along the lines of humanities free will. They fight to keep the freedom so that all men and women have freedom without a strict dictator code set by their enemies the templars. They seek to find out the truth and to preserve mankinds free will.
Then again I may be wrong so checking out AC's wiki site may help
BTW RedDevil00 pic is SICK Ezio is and forever shall be an Assassin Legend